In the field of horology, the plates on which movements are arranged generally contain a thread, i.e. an upper edge which is provided for attaching the movement to the case. To this end, the thread can be secured with the aid of clamps and screws which are inserted into threaded holes and thus pressed against a shoulder of the case middle. Although this type of attachment has the advantage of being reliable and reversible, it also has the drawback of being relatively complex to implement due to the various manipulations required for the screws and clamps, which seriously affects productivity and makes this method unsuitable for the manufacture of watches intended for mass production.
US Patent Publication No 2002/0131332 by the Applicant describes an alternative method for attaching a movement which does not require the use of a thread or clamps, but still uses a securing screw. A first series of screws is inserted in through holes arranged at the periphery of the movement to assemble the latter to a bezel, while the back cover is then assembled to the same bezel using a second series of screws. Thus, although the central element for this type of attachment is no longer the case middle, but the bezel of a watch, the same drawbacks in terms of productivity nonetheless remain, and are even accentuated due to the increased number of screwing operations required during assembly of the movement.
This is why press-fit attachment methods are sometimes preferred, usually with the aid of intermediate parts such as casing rings, which also allow small calibrated movements to be fitted in larger cases.
The solution described in EP Patent No 1046967 by the Applicant concerns, for example, such a method for attaching a movement with the aid of a casing ring made of a plastically deformable material intended to allow the radial or axial adjustment of the movement in a determined position with respect to the case.
Regardless of the attachment method chosen, the lower rim of the thread is always made to abut against a portion of the case middle, and the shape of the case back, which may or may not be integral with the case middle, matches that of the lower portion of the movement to within machining tolerances. U.S. Pat. No. 4,558,955 describes such a solution for attaching a movement, wherein the movement is directly accommodated in a hollow in a one-piece case middle-back cover and is held axially between the bottom of this hollow, against which it abuts, and the dial, retained by the crystal. The drawback of this type of attachment method is that any deformation of the back cover, particularly during subaquatic use, is directly replicated on the plate, and the latter is particularly sensitive.
To prevent any inadvertent deformation of the constituent parts of the watch, an alternative consists, especially for plastic watches, in using reinforced structures containing, for example, frames made of metal or other types of materials for strengthening the case. This alternative cannot, however, be envisaged if it is wished to maintain the attractive features of the case, particularly if the case is required to remain at least partially transparent.
Otherwise, there are also known structures with a double back cover for watches accommodating other internal modules in addition to the movement. EP Patent No 0670532 concerns, for example, a watch provided with a pressure sensor and describes a movement assembled in a conventional manner to a case middle with the aid of a casing ring, while an inner back accommodates the pressure sensor, covered by the back cover of the case. Such a structure has the drawback, however, of adding considerable thickness to the case and also slowing down production time due to the doubling of assembly operations for each of the backs.
There therefore exists a need for a method and a device for attaching a movement inside a case that is free of these known limitations.